1. Field of the Invention
This invention relates to novel heat-curable resin compositions suitable for the preparation of powder paints.
2. Description of the Prior Art
Powder paints have attracted attention as pollution-free paints, and have recently come into increasing acceptance because of their various advantages. For example, high coating yields can be obtained because the paints can be recovered and reused, and in one coating operation a coating having a desired coating thickness ranging from about 30 microns to about 500 microns can be obtained.
Various thermoplastic resins and thermosetting resins have been considered as materials for powder paints. Of these, heat-curable polyester resins and heat-curable polyester-type resin compositions are widely expected to be suitable as materials for powder paints that provide cured coatings having superior weatherability, mechanical strength, and chemical properties.
It has been known heretofore that heat-curable resin compositions consisting of polyesters containing carboxyl groups and polyoxirane compounds can be used as materials for powder paints. For instance, British Pat. No. 1,381,262 discloses that a heat-curable resin composition composed of a mixture of (1) a solid modified polyester obtained by reacting a polyester having a hydroxyl number of 15 to 50 (obtained from an acid component and an alcohol component by a conventional polyester-forming reaction) with an anhydride of a polybasic organic carboxylic acid and (2) an epoxy resin containing at least 2 epoxy groups on the average per molecule can be used as a material for powder paints. However, with the disclosed method, a heat-curable resin composition having the various properties required of a material for powder paint is difficult to obtain on a commercial scale in a reproducible, stable manner.
Generally, the degree of polymerization of polyesters prepared by a polyester-forming reaction or a polycondensation reaction is controlled by, for example, the following methods.
(1) A method wherein the end point of the reaction is determined by measuring the melt viscosity of the polyester, that is, the torque or power required to stir the same.
(2) A method wherein the end point of the reaction is determined by measuring the carboxyl content or hydroxyl content of the polyester collected from the reactor.
(3) A method wherein the end point of the reaction is determined by the amount of the alcohol or water removed from the reaction system.
(4) A method wherein the end point of the reaction is determined by maintaining the reaction conditions constant.
However, it is difficult by these methods to produce a polyester having a relatively low average degree of polymerization of about 3 to about 50 in a reproducible, stable manner on a commercial scale which is suitable for the preparation of a heat-curable resin composition for powder paint.
Moreover, in the addition reaction between a hydroxyl-containing polyester and a polybasic organic carboxylic acid anhydride carried out at a temperature of about 180.degree. C to about 250.degree. C, esterification simultaneously proceeds. Hence, the average degree of polymerization and the carboxyl group content of the polyester are difficult to adjust to constant values.
On the other hand, U.S. Pat. No. 3,954,901 discloses that a polyester having a high degree of polymerization can be depolymerized using a substantially non-volatile alcohol and/or an ester containing at least one alcoholic hydroxyl group per molecule to produce a polyester resin having a low degree of polymerization suitable for the preparation of heat-curable resin compositions for powder paints.